What are the dimensions of the tank? With the fish you're talking about footprint is more important than total litres.
Make sure the ornate has a good head start on the oscars, otherwise it will end up as spaghetti. You might also want to train it to hand feed from an early age, otherwise it could be hard to get food to it with out the oscars getting it first.
Most people here will shun you for mixing a frontosa with those fish, but plenty of people do. Keep the pH around 7 and it should be alright. The biggest problem would be the difference win growth rate between the front and the oscars.
And for the substrate, just about anything will be fine with oscars, but the red grit you're talking about would be better for the bichir than pea gravel.